Note: This issue relates to MariaDB but might also happen with MySQL. The key thing seems to be a slightly more rigorous permissions config under El Capitan.

I recently struck an issue where MariaDB would not start under El Capitan. Frustratingly, the only error produced was this: “ERROR!”

The fix for me was simple:

Finder > Go > Go To Folder…
/usr/local/var/
Click mysql and File > Get Info
Click the Unlock Icon at bottom right, and authenticate. Then change the persmissions to ‘Read and Write’ for everyone.
Finally, click the gear icon near the bottom left, and click ‘Apply to Enclosed Items.’

MariaDB should now start correctly.